MontCo Fence Blog

Fence Post Depth and You: How to Decide How Deep to Dig a Fence Post

The rule of thumb is that one-third of the fence post should be below ground and two-thirds above ground. So, you need poles nine feet long for fence posts to be six feet above the ground.

Fence post depth depends on the type of substrate, fencing material used, and the purpose of the fence. Let's explore how deep you need to go, and when.

Fencing Material

The most common materials used for fence posts are wood, aluminum, and PVC. Most people use the same material for the fence panels as they do for posts, but chain link or diamond mesh fences are also options. Preparation of the hole, and its depth, are the same regardless of the post material used.

Substrate Type Affects Fence Post Depth

The type of soil influences the fence post depth you need. It is advisable to dig a little deeper than the one-third of fence post height in sandier soil. The one-third rule excludes the six inches deeper you have to go to allow space for the adding of gravel at the base of the hole for extra stability.

Hole Size and Shape

There are two general rules:

  1. The diameter of your hole should be roughly three times the diameter of the post.
  2. The hole itself should be bell-shaped (wider at the bottom than at the top) rather than carrot-shaped.

The bell-shape prevents "frost heave" – the way posts work themselves out of the ground after several frost-thaw cycles. In colder climates, dig your hole to below the frost line for the same reason.

The frost line is the first point below grade (ground level) at which freezing will not occur. Local building regulations normally specify a depth of at least 30 inches.  If you live in a colder climate, this is the depth you would have to dig – even for a fence only four feet high.

Measuring

Because your hole is three times the width of your fence post, you need to measure from the center of one hole to the center of the next hole to ensure correct fence post placement. You can mark out the line of your fence in advance, but re-measure carefully when positioning where your next hole needs to be dug.

Align center of the post with the center of the hole in each case. 

Cement Anchors

Once your hole is at the correct depth, making allowances for the gravel at the base, add your gravel. Position your pole on top of the gravel, and ensure that it is standing straight using a spirit level.

With the pole correctly positioned, add already mixed concrete into the hole, and fill it a little more than halfway. Secure the pole in place with planks until the concrete dries, then cover it with soil, stamping it down as you go.

Some Holes Are Deeper Than Others

Gateposts and corner posts take greater strain than other fence posts, so dig these deeper, to about half the above-ground length of the post.

Deeper holes for fence posts are needed if you're using heavier than normal fence panels.

Call The Experts

If you would rather the fencing experts did all the hard work of getting down to the correct fence post depth, contact us, and we'll be happy to give advice and install your fence for you.

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